In digital pathology and particular in the case of whole slide scanning, specimens are sliced and imaged for analysis purposes as well as teaching purposes. Line sensors may be used for scanning a whole tissue slide. These slide scanners may perform a continuous mechanical scanning, thereby reducing stitching problems and allowing for the use of so-called time delay integration (TDI) line sensors in order to accommodate for low brightness of the illumination, as for example disclosed in WO 01/84209 A2.
Before the actual scanning the optimum focus position may be determined at a number of positions on the slide. This results in a “focus map”. This procedure can be used when the axial position of the tissue layer may vary with several micrometers across the slide, as depicted in FIG. 1 (to avoid any doubt, it should noted here that the term axial refers herein to a direction along the axis 7 shown in FIG. 2).
The variation of the tissue layer may thus be more than the focal depth of the microscope objective. During scanning the focus position of the objection is set on a trajectory that interpolates between the measured optimum focus settings on the selected measurement locations. However, this procedure may be both prone to errors and be also time-consuming, thereby limiting the throughput of the system.
In known autofocus imaging systems it may not be possible to determine the sign of the focus error (above or below focus), i.e. the focus error signal may not be polar. This may be disadvantageous for a continuous autofocus system that needs permanent updates on the optimum focus setting.
WO 2005/010495 A2 describes a system and a method for generating digital images of a microscope slide, the microscope comprising a main camera and a focus camera which is tilted with respect to the optical axis.